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//! A "mutex" which only supports try_lock
//!
//! As a futures library the eventual call to `epoll` should be the only thing
//! that ever blocks, so this is assisted with a fast user-space implementation
//! of a lock that can only have a `try_lock` operation.
use std::cell::UnsafeCell;
use std::ops::{Deref, DerefMut};
use std::sync::atomic::Ordering::{Acquire, Release};
use std::sync::atomic::AtomicBool;
/// A "mutex" around a value, similar to `std::sync::Mutex<T>`.
///
/// This lock only supports the `try_lock` operation, however, and does not
/// implement poisoning.
pub struct Lock<T> {
locked: AtomicBool,
data: UnsafeCell<T>,
}
/// Sentinel representing an acquired lock through which the data can be
/// accessed.
pub struct TryLock<'a, T: 'a> {
__ptr: &'a Lock<T>,
}
unsafe impl<T: Send> Send for Lock<T> {}
unsafe impl<T: Send> Sync for Lock<T> {}
impl<T> Lock<T> {
/// Creates a new lock around the given value.
pub fn new(t: T) -> Lock<T> {
Lock {
locked: AtomicBool::new(false),
data: UnsafeCell::new(t),
}
}
/// Attempts to acquire this lock, returning whether the lock was acquired or
/// not.
///
/// If `Some` is returned then the data this lock protects can be accessed
/// through the sentinel. This sentinel allows both mutable and immutable
/// access.
///
/// If `None` is returned then the lock is already locked, either elsewhere
/// on this thread or on another thread.
pub fn try_lock(&self) -> Option<TryLock<T>> {
if !self.locked.swap(true, Acquire) {
Some(TryLock { __ptr: self })
} else {
None
}
}
}
impl<'a, T> Deref for TryLock<'a, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.__ptr.data.get() }
}
}
impl<'a, T> DerefMut for TryLock<'a, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.__ptr.data.get() }
}
}
impl<'a, T> Drop for TryLock<'a, T> {
fn drop(&mut self) {
self.__ptr.locked.store(false, Release);
}
}
#[cfg(test)]
mod tests {
use super::Lock;
#[test]
fn smoke() {
let a = Lock::new(1);
let mut a1 = a.try_lock().unwrap();
assert!(a.try_lock().is_none());
assert_eq!(*a1, 1);
*a1 = 2;
drop(a1);
assert_eq!(*a.try_lock().unwrap(), 2);
assert_eq!(*a.try_lock().unwrap(), 2);
}
}